Autonomous system numbers, explained

What an AS number is

An autonomous system number identifies one network on the global routing table. An autonomous system is a group of IP prefixes operated under one routing policy, and the number is the integer that names that group when networks exchange routes. Our IP lookup shows the number and the registry facts.

The registration country of a number is not the country where the traffic flows. It comes from the registry record, which the assigning regional internet registry maintains; it can lag a transfer, name a holding company, or point to a headquarters while the network announces prefixes used on several continents. Read it as registry metadata, not as a service map. For the short definition, see AS number; BGP covers the protocol that carries the announcements.

How a number appears in a route

A route announcement carries a prefix, an origin AS number and a path of AS numbers. BGP speakers exchange these announcements, and the AS path records the sequence a route has traversed, so the same prefix can arrive with different paths at different vantage points.

In a received AS path, the rightmost number is normally the origin, the network that generated the announcement. The leftmost is the neighbor that sent it to the observer. Prepending means repeating a number to make a path look longer; it is a traffic-engineering tool, not evidence of physical distance.

Looking glasses and route collectors show one view at one moment. Route aggregation can hide intermediate networks, and a collector that is not on the path cannot show every hop. Treat a single view as evidence of reachability, not as the whole route.

Two-byte, four-byte and private numbers

Early AS numbers were 16-bit. RFC 6793 extended the field to 32 bits so the supply would not run out. A 32-bit number is written as a plain decimal integer, or in asdot notation as two 16-bit parts, so 65536 becomes 1.0. Modern BGP treats both widths as one space; the old two-byte distinction is historical.

Some numbers are not for public routing. 64512 to 65534 and 4200000000 to 4294967294 are reserved for private use (RFC 6996). 64496 to 64511 and 65536 to 65551 are reserved for documentation (RFC 5398). 0, 65535 and 4294967295 are reserved (RFC 7607, RFC 7300). 23456 is AS_TRANS, a placeholder used when a four-byte number must cross an old two-byte session.

A private number in a public path usually means a leak, a misconfiguration, or a view that includes a provider's internal topology. It is not a hidden public network. A 23456 in a path is a translation marker, not a real origin.

Reading a lookup without over-reading it

An ASN lookup returns registry facts: the assigning registry, the allocation or assignment, a registered contact, a country and dates. The five regional internet registries are AFRINIC, APNIC, ARIN, LACNIC and RIPE NCC. IANA allocates blocks to them, and they assign numbers to operators under regional policy.

The organization name behind a number changes when a network is sold or renamed, while the number stays. Read the name as a registry snapshot, not as a statement about ownership, service quality or a commercial relationship. One number can announce many prefixes, and one prefix can be announced by more than one number. Neither fact is automatically a fault; both are reasons to check the routing policy as well as the registry.

What an AS number does not tell you

It does not measure latency, loss, jitter or available bandwidth. AS path length is one input to BGP best-path selection, not a distance and not a quality score; a two-AS path can be slower than a five-AS path.

It does not prove that an announcement is authorized. BGP itself has no built-in origin authentication (RFC 4271), so operators use route filters, Internet Routing Registry objects and RPKI to decide which origin may announce a prefix. A route can be visible and still be invalid.

It does not identify our prefixes, our suppliers or a customer. This page explains the identifier; commercial details belong in a quote conversation.

Routing security, in two mechanisms

An Internet Routing Registry route object is a registry statement that a given AS originates a prefix, written in the Routing Policy Specification Language (RFC 2622). Its value depends on the object and on the filters that use it. An RPKI route origin authorization is a signed statement that a prefix may be originated by a number, up to a maximum length (RFC 6480, RFC 6482). Validating routers can drop invalid announcements, but partial deployment means a missing authorization is not proof of wrongdoing.

Ask any provider which filters it applies and how it handles invalids, and ask for the method rather than a badge. We keep routing views out of public pages and discuss path design in quotes.

Why the number matters for China-bound routes

For an IP service into China, the path crosses several networks, and an AS number seen in a traceroute is a forwarding hop, not the whole commercial path. The same destination can be reached through different international routes with different latency and loss. Private-line products such as IEPL and IPLC forward Ethernet or time-division capacity, so an IP traceroute may not reveal the underlying path at all. A clean traceroute is not proof of a private line; a contract and a handoff diagram are.

Our China pages describe route types and the questions to scope. They do not publish a network map or a list of prefixes. China routing explained covers the service view; IEPL vs IPLC covers the Layer 2 vocabulary.

Common mistakes when reading an IP result

Treating the registry country as the traffic's location. Treating a long AS path as slow, or a short one as fast. Treating a name in Whois as the current operator. Treating a private number in a looking glass as public. Treating a multi-origin prefix as an attack. Treating a visible route as an authorized route.

Use two vantage points, check the timestamp, compare the prefix and origin, and read the registry record through RDAP rather than a copied Whois dump. For an address you control, the IP and ASN lookup shows the number, and the CIDR calculator helps confirm the prefix boundaries. Keep a short record of what you checked and when. Routing changes quickly, and a note with a timestamp is worth more than a screenshot without one.